Welding is indispensable for the construction of steel structures and large-heat-input welding is extensively applied to cut down construction cost and increase construction efficiency. Particularly, as steel plate thicknesses increase, the time required for welding increases greatly and, therefore, the need to perform welding with the maximum large-heat-input also increases.
However, application of large-heat-input welding lowers the toughness of the heat-affected zone (HAZ), increases the width of the HAZ and, as a result, lowers the toughness against brittle fracture.
Because of this, steels which are resistant to a drop in the fracture toughness in the HAZ even when large-heat-input welding is applied were invented as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 06-88161 and 60-245768.
Having improved fracture toughness or greater resistance to brittle fracture, the steels according to the above inventions seldom undergo brittle fracture under ordinary service conditions. If, however, a brittle fracture occurs in an earthquakes or other accident and disasters like a collision between structures, there are risks that brittle fractures propagate through the HAZ and cause massive fractures.
It has conventionally been considered that, with TMCP steel plates of approximately 25 mm in thickness, brittle fracture diverts from welded joints to the base metal because of the residual stress in the weld zone. Therefore, it has been considered that brittle cracks occurring in welded joints would be stopped in the base metal so long as the base metal has adequate crack arrestability.
With the upsizing of steel structures, thicker steel plates have come to be used. Because the use of thicker plates is conducive to structure simplification too, high tensile strength steel plates offering high design stresses are finding increasing use.
The full-size breakdown test conducted by the inventors by using an 8000-ton large-sized tester revealed that brittle cracks developed in thicker steel plates of the type described above propagate along the heat-affected zone of welded joints without diverting to the base metal.
In the brittle fracture test conducted by the inventors, a stiffener (reinforcement) 3 fillet-welded to a steel plate 1, not greater than 50 mm in thickness, so as to intersect a butt-welded joint thereof as shown in FIG. 1 often arrested the propagation of brittle crack developed in the steel plate 1 and thereby prevented the fracture thereof.
In thicker plates, such as those, for example, exceeding 50 mm and reaching approximately 70 mm in thickness, brittle cracks sometimes propagated along the HAZ or weld metal without diverting to the base metal despite the provision of the stiffener 3.